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FGD stands for Forge Game Data. It is the file extension for Hammer's game definition files. They define all of the entities of a game so mappers can select them from within the editor. It is a key point to understand that an FGD is nothing more than a reference. You cannot create or modify entities by editing a FGD, you merely give Hammer different information about what it expects to find within the game. Sometimes editing reveals hidden or unused features or even entities, but they were always there and could be used even without the updated FGD. For Custom FGD files to work in hammer, they must be added through tools > options.

Note.png Note: FGD's are loaded in alphabetical order. If you do not use @include "base.fgd" at the top of your FGD and it is loaded before base.fgd you will encounter errors.

To do: Source 2 adds some extra features to the FGD format, document those.


While Hammer was originally called Worldcraft, it was developed under the name The Forge (hence the name Forge Game Data). However, due to trademark issues, the name Forge couldn't be used for the final version of Hammer. Still, the file extension stayed.

File format

The FGD file follows a fairly simple format. It is a script file that sets up entity structures and relationships for Hammer. The various parts of the Half-Life 2 FGD (found in your SDK binary directory, for example: (path_to_steam)/SteamApps/common/Source SDK Base 2013 Singleplayer/bin/) are explained below.

//====== Copyright © 1996-2005, Valve Corporation, All rights reserved. =======
// Purpose: Half-Life 2 game definition file (.fgd) 

Comments are defined simply by starting a line with //. They can be preceded by spaces or tabs.

@include "base.fgd"

If the game you are writing your FGD for has a lot in common with another game (Half-Life 2 and Counter-Strike: Source, for example), you can include a file that has all of the common structures defined in it. The FGDs for Half-Life 2 and Counter-Strike: Source both include the base.fgd file, and the FGD for Half-Life 2 Deathmatch includes the halflife2.fgd file.

@BaseClass base(BaseNPC) = TalkNPC
	UseSentence(string) : "Use Sentence"
	UnUseSentence(string) : "Un-Use Sentence"

A BaseClass is used to setup structures that are used by several different entities. They are referenced in an entity structure by adding base(BaseClassName) to the main definition line of the structure. The BaseClass structure is defined just like a normal entity, in all respects. The only difference is that it doesn't appear in the entity lists in Hammer. (We'll discuss the complete entity structure below).

@PointClass base(Targetname, Origin) studio("path/model.mdl") = example_entity : "example"
	spawnflags(flags) =
		32 : "A flag" : 0       // 0 means the flag is not-ticked by default
		64 : "Another flag" : 1 // 1 means the flag is ticked by default

	foobarname(string) : "Name" : : "Name of foobar"
	foobargroup(string) : "Group" : "Squad1" : "Name of foobar group"
	foo(float) "Floating point number" : "100.7" : "Decimal points = fun"
	something(integer) readonly: "first number" : 0 : "This is a number which can't be manually edited"
	something2(choices) : "second number" : 0 : "Your choice of numbers!" =
		0 : "Default"
		1 : "Something"
		2 : "Another Thing"

	// Outputs
	output OnSomethingHappened(void) : "Fires when something happens"
	output OnSomethingElse(void) : "Fires when something else happens"

	// Inputs
	input DoSomething(void) : "Do something"

Above is a generic example of an entity structure as defined in the FGD. Let's break it down bit by bit, starting with the first line:

  • @PointClass - The class type of an entity tells Hammer how this entity can be placed.
  • @PointClass - This entity exists at a certain non-arbitrary point. It is typically referred to as a "point entity". The entities are placed within Hammer by using the Entity tool (Shift+E).
  • @NPCClass - This is a special form of point entity tailored for NPC (non-player character) entities. It is useful in conjunction with the npcclass property type, below.
  • @SolidClass - This entity's area is defined by the solid (also referred to as a brush) that it is attached to. It is typically referred to as a "brush entity" or "solid entity".
  • @KeyFrameClass - Used for move_rope and keyframe_rope. This causes the NextKey property to be linked up when the entity is copied.
  • @MoveClass - Used for path_track and similar entities. This causes the target property to be linked up when the entity is copied.
  • @FilterClass - One of the special filter classes used to define what entities will be able to interact with another in some way. This mainly causes the entity to be shown in properties with the filterclass type.
  • base(Targetname, Origin) - Things between the type declaration and the "=" character help to define properties of the entity and how it will act and be displayed in Hammer. There are a number of different things that can used here. (More than one of these can be used, each separated by a space.)
  • base( BaseClass1, BaseClass2, ... ) - This lets you attach previously defined BaseClasses (see above) to an entity. You can specify multiple BaseClasses, separated by a comma.
  • bbox( min,max ) - Sets the size of the entity's bounding box.
  • obb( min,max ) - Identical to bbox, but oriented to the entity's angles.
  • color( rrr ggg bbb ) - This setting will change the color of the wireframe box in the Hammer 2D views. If this isn't present, the color will default to magenta. The values specified here are the RGB values of a color, and each number has a range from 0 to 255.
  • iconsprite( "path/sprite.vmt" ) - If this is used, the specified sprite will be shown in the Hammer 3D view instead of a flat-shaded colored box. This will work along-side the studio() or studioprop() commands. If no sprite name is set, use the model property. scale, rendermode, _light and angles properties affect the sprite.
  • sidelist( sides ) - Highlight brush faces listed in the given property (as a space-seperated ID list). If not specified the property is sides.
  • sphere( propertyname ) - If an entity has a radius of effect, like a sound for example, a sphere will be displayed in Hammer's 2D and 3D views. You need to specify the property that will control the sphere size. If no property is specified, it will look for a radius property.
  • studioprop( "path/model.mdl" ) - If this is used, the entity will be displayed in the 3D view as the specified model. If no model is specified, the value of the entity's "model" property will be used, if available. Multiple models can be defined.
Note.png Note: If you have an entity with the "angles" property that you want to be able to rotate in Hammer using the mouse (as opposed to only through property editing) you may need to add this modifier.
Note.png Note: The appearance is affected by the skin and rendercolor properties, similar to prop_dynamic.
  • studio( "path/model.mdl" ) - Identical to studioprop(), but the bounding box around the entity will ignore this model. This is useful for entities that don't render the model ingame.
  • lightprop( "path/model.mdl" ) - Identical to studioprop(), except that the pitch of the model is inverted.
  • line( color, start_key, start_value, end_key, end_value ) - Draws a line between two entities. The value properties in this entity give the names to look for in the key property on other entities. key is usually set to targetname. The color sets the color of the line when the entity is not selected. The second entity defaults to this one if not set.
  • cylinder( color, start_key, start_name, start_radius, end_key, end_value, end_radius ) - Draw a cylinder between two entities. This is similar to line(), but with the addition of two radius properties that are looked up on the target entities. These define the size of the start and end of the cylinder.
  • lightcone( inner_fov, outer_fov, color, pitch_scale ) - Renders the cone used on light_spot entities. inner_fov is the key for the innermost cone section, outer_fov is the outermost. pitch_scale allows inverting the pitch angle when rendering the cone. Values are taken from _inner_cone, _cone, _light respectively if not specified. This reads many other values corresponding to light_spot properties.
  • frustum( fov,near,far,color, pitch_scale ) - Creates a rectangular cone extending from the entity. FOV defines the spread angle (0-180). Near and far define at what distances will be highlighted. The color value defines what color the cone will be shown with. Pitch_scale allows inverting the pitch angle when rendering the cone. The first four values must be property names, the last is a literal. If not specified, values are taken from _fov, _nearplane, _farplane and _light respectively. pitch_scale is set to -1.
  • halfgridsnap - When moving this entity, it will snap to half the current grid size. This is somewhat special as it takes no arguments or parentheses.
  • wirebox( min,max ) - Draws a bounding box for two properties. origin() helpers should be defined as well to allow moving the points.
  • size( -x,-y,-z,+x,+y,+z ) - Defines the size of the default cube used when no model or sprite is specified.
  • origin( property ) - Allows positioning a vector property in the map.
  • vecline( property ) - Allows positioning a vector property in the map. This also draws a line from the entity to the position.
  • axis( property ) - Allows positioning two points joined by a line in the map. The property value is set to "x1 y1 z1, x2 y2 z2".
  • worldtext() (only in Counter-Strike: Global Offensive) (also in Portal 2: Community Edition) - Displays the contents of the "message" keyvalue in the 3D viewport.

The following helpers take no arguments, and are special-cased for specific entity types:

  • decal() - Renders decals on nearby surfaces. This uses the texture property to set the material to use.
  • overlay() - Renders overlays on a surface. (info_overlay).
  • overlay_transition() - Renders overlays on the intersections between water and the shore. (info_overlay_transition).
  • light() - Present on light, unknown use. To do: Figure out what this does.
  • sprite() - Renders the sprite material specified in the "model" keyvalue (env_sprite and variants). For entity icons, use iconsprite.
  • sweptplayerhull() - Draws 32x32x72 sized rectangular prisms at two points (point0 and point1), then links corners to show the space needed for one rectangle to move to the other's position. This also adds origin() helpers for those properties.
  • instance() - Renders the instance in the map. It also generates additional properties dynamically corresponding to the instance parameters.
  • quadbounds() - Used for func_breakable_surf. Automatically sets the 4 corners of the textured face on save. To do: Does this also control the "error" keyvalue?
  • example_entity : "example" - This is the entity's name followed by a description. The description is displayed in Hammer when you click on the Help button inside the entity property dialog. For visual ease, the description can span multiple lines by joining "blocks of text" with the plus (+) character. For example:

@PointClass = example_entity :

"This is an example description for"+
"this example entity. It will appear"+
" in the help dialog for this entity"


entity properties go here


  • entity properties - Everything between the main set of [ / ] brackets is used to define the entity's properties, including their inputs and outputs. Individual property structures consist of a name, a type declaration, a display name, a default value, and a description. The most common properties are:
  • string - This creates a property of the string type.
name(string) : "Display name" : "Default" : "Description"
  • integer - This creates a property of the integer type.
name(integer) : "Display name" : 1 : "Description"
  • float - This creates a property of the float type. Although it deals with numbers, the structure of it is similar to the string type. The default value must have quotes around it.
name(float) : "Display name" : "1.5" : "Description"
  • boolean (in all games since Alien Swarm)(also in Garry's Mod) - This creates a property of the boolean type, with a dropdown for yes/no. For older versions of Hammer, use choices instead.
name(boolean) : "Display name" : 1 : "Description"
There are also two common special case property types, choices and flags, that follow a slightly different format.
  • choices - A property of this type lets you setup a number of distinct choices. Their format is similar to the other types:

name(choices) : "Display name" : "1" =

0 : "something"
1 : "something else (default)"
2 : "something completely different"


You can also use strings (or floats) as values, instead of integers, like this:

name(choices) : "Display name" : "models/something02.mdl" =

"models/something01.mdl" : "something"
"models/something02.mdl" : "something else (default)"
"models/something03.mdl" : "something completely different"


  • flags - The flags property type lets you setup what will appear in the Flags portion of the entity property dialog. It is setup similar to the choices property type. The flags are all powers of 2 - 20, 21, 22, etc, and their values are either 0 (off) or 1 (on). If no default is specified for a flag, it is considered to be off.

spawnflags(flags) =

1 : "something clever" : 1
2 : "something else" : 0
4 : "you said what now?" : 0
8 : "nothing" : 1


Note that spawnflags is always the name of this property. Creating a flags-type property with any other name will result in it appearing as a normal integer keyvalue.
There are also a number of special purpose property types that modify the entity properties dialog UI to allow for easy browsing for files or easier manipulation of complex properties (like colors or angles).
  • axis - adds a relative 2 point axis helper
  • angle - adds an angle widget for this property to the entity dialog UI.
  • angle_negative_pitch - identical to angle, except the pitch is inverted.
  • color255 - adds a button that brings up the color choosing UI, which takes the color you select and translated it into the three number RGB value. Allows extra parameters (i.e. brightness)
  • color1 - adds a color button, but uses a float [0,1] to represent RGB. Allows extra parameters (i.e. brightness)
  • filterclass - marks property as being the name of the filter to use
  • instance_file - adds a button that brings up the file browser, letting you browse for instance files.
  • instance_parm - used in func_instance_parms to define fixup variables.
  • instance_variable - used in func_instance to set fixup variables.
  • material - adds a button that brings up the material browser.
  • node_dest - adds an eyedropper to select a node in the 3d view
  • node_id (in all games since Alien Swarm) - On nodes, this is used for the Node ID keyvalue, to automatically increment it with each consecutive node placed. Does not appear to function when used on other entities.
  • npcclass - adds a drop-down selection list populated by entities of the NPCClass type.
  • origin - origin
  • particlesystem (in all games since Alien Swarm) - adds a button that brings up the particle browser, letting you browse for particle systems.
    Bug.png Bug: The particle browser cannot read files from VPKs! You will need to extract the game's PCF files for anything to appear.
  • pointentityclass - adds a drop-down selection list populated by entities of the PointClass type.
  • scene - adds a button that brings up the sound browser, letting you browse for scene files.
  • script (in all games since Left 4 Dead 2) - adds a button that brings up the file browser, letting you browse for VScripts.
  • scriptlist (in all games since Left 4 Dead 2) - adds a button that brings up a list of VScripts, with buttons to add/remove scripts and open each file.
  • sidelist - adds a side selection eyedropper that allows you to choose sides (multiple with ctrl).
  • sound - adds a button that brings up the sound browser, letting you browse for soundscripts or raw sounds.
  • sprite - identical to material, except it will automatically replace your search filter with sprites/ when opening the material browser, and will add .vmt to the end of the material name.
  • decal - identical to material, except it will automatically replace your search filter with decals/ when opening the material browser.
  • studio - adds a button that brings up the model browser.
  • target_destination - marks property as another entity's targetname.
  • target_name_or_class - marks property as another entity's targetname or classname.
  • target_source - marks property as being the name that other entities may target.
  • vecline - adds an absolute 1 point axis helper, similar to the origin marker.
  • vector - 3d vector

Other file sections

Material Exclusion Lists

These lists define paths that Hammer's Material Browser will not use when presenting you with a palette of textures to choose from. It should have no effect on what files are actually available to a custom map.

Note.png Note: Any folder called models/ will always be excluded, regardless of what is set here.
    // Names of the sub-directories we don't want to load materials from

Customizing auto-visgroups

This permits customizing the automatic Visgroups tab of the Filter Control toolbar. The first title is the name of the 'parent', and the next is the 'children'. Finally comes a list of entity classes that will be placed in the visgroup. If the parent already exists, the new entry will be merged with the previous (including the default list of groups). This permits creating trees with multiple levels of grouping. If a visgroup becomes entirely empty, it will not appear in the list.

Note.png Note: You are able to add entities to existing groups. If the "Parent" of the autovisgroup is the name of an existing group, like "World Details" you can have your own entities be listed along "Props" and "Func Detail". For example adding func_brush to that list.
@AutoVisGroup = "Parent"
    "Child 1"
        "entity name 1"
        "entity name 2"
    "Child 2"
        "entity name 3"
        "entity name 4"


@AutoVisGroup = "Brushes"
    "Tool Brushes"

@AutoVisGroup = "Tool Brushes"
    "Vis Clusters"

External links